1. Field of the Invention
The present invention relates to an image forming apparatus that is capable of preventing an image defect caused by variation in surface speed of a photosensitive drum due to off-centering of a rotating shaft of the photosensitive drum.
2. Description of the Related Art
In electrophotographic image forming apparatuses including a copy machine, a multifunction peripheral, and a facsimile machine, photosensitive drums and an intermediate transfer belt which carry toner images are required to be driven at a constant surface speed. This is because, first, variation in the surface speed of the photosensitive drum causes deviation of a laser irradiation position from an original proper position to be irradiated. Secondly, in a primary transfer process for transferring a toner image formed on the photosensitive drum onto the intermediate transfer belt, if there occurs an AC current-like variation in the difference of surface speed between the photosensitive drum and the intermediate transfer belt, the position of the toner image transferred onto the intermediate transfer belt deviates from the original proper position where the toner image is to be transferred. That is, in a case where the photosensitive drum and the intermediate transfer belt are not driven at a constant surface speed, an image defect, such as image color shift caused by positional displacement between images of respective colors, or a periodical positional displacement called banding, occurs on an image which is finally formed on a recording sheet.
To overcome the above-mentioned problem, in the control of driving the photosensitive drum and the intermediate transfer belt, the feedback-control of the speed of a motor as a drive source is performed, based on results of detection by various speed detection sensors and the like, whereby highly-accurate speed constancy is ensured. Further, as the drive motor, a brushless DC motor (hereinafter referred to as the “BLDC motor”) is often used because of low-cost, quietness, and high efficiency. In recent years, for the speed feedback control using the BLDC motor, a method is employed in which a rotary encoder is arranged on a drum shaft, and the CPU controls the BLDC motor to rotate the drum shaft at a constant speed.
However, in the above-mentioned speed feedback control, although the rotational speed of the drum shaft is detected, the surface speed of the photosensitive drum is not detected, and hence there is a case where the surface speed of the photosensitive drum fails to be constant e.g. due to off-centering of the drum shaft and low accuracy of the diameter of a roller. Similarly, in the intermediate transfer belt as well, the same problem is caused e.g. by off-centering of a rotating shaft of an intermediate transfer belt-driving roller which drives the intermediate transfer belt, low accuracy of the diameter of the roller, and variation in thickness of the intermediate transfer belt.
On the other hand, factors causing the image defects include mutual interference caused by friction between the surface of the photosensitive drum and the transfer surface of the intermediate transfer belt. This is caused because a speed variation occurring in one of the photosensitive drum and the intermediate transfer belt has influence on the other. As another factor, there may be mentioned an occurrence of an unplanned change in load on the intermediate transfer belt during secondary transfer of a toner image carried on the intermediate transfer belt onto a recording sheet, especially when the recording sheet is thick paper. This causes a high-frequency speed variation, and this speed variation causes positional displacement in the primary transfer. As described above, there are various factors causing the image defects, and it is very difficult to eliminate all of the factors causing the defects.
To cope with this, as described in Japanese Patent Laid-Open Publication No. 2002-333752, there has been developed a technique in which an image transfer roller (which corresponds to the intermediate transfer belt) causes an image roller (which corresponds to the photosensitive drum) to be friction-driven. According to this technique, the following advantages can be obtained: First, images on the photosensitive drums become an image on the intermediate transfer belt, and hence by forming the image on the intermediate transfer belt with reference to respective positions on the photosensitive drums, the influence of irregular rotation of the photosensitive drums is reduced. Further, secondly, even when the speed of the intermediate transfer belt is varied e.g. due to an impact generated upon entrance of a recording sheet into a secondary transfer section of the intermediate transfer belt, matching of respective images on the photosensitive drums with the image on the intermediate transfer belt can be ensured, and hence image defects in the primary transfer are less liable to occur.
However, to obtain the first advantage, it is important to form an image with reference to the rotational position of each photosensitive drum. To this end, as described in Japanese Patent Laid-Open Publication No. H08-99437, there has been developed a technique for performing exposure control in synchronism with an amount of rotational movement of the drum (see e.g. Japanese Patent Laid-Open Publication No. H08-99437). Further, there has also been developed a technique for directly detecting a speed of the surface of the photosensitive drum (see e.g. Japanese Patent Laid-Open Publication No. 2007-156194).
The technique disclosed in Japanese Patent Laid-Open Publication No. H08-99437 is for performing the exposure control in synchronism with an amount of rotational movement of the photosensitive drum. However, in this technique, assuming that the rotating shaft is arranged on the center position of the photosensitive drum and that the diameter of the photosensitive drum is accurately the same as designed, it is possible to obtain a value equivalent to that obtained by detecting an amount of surface movement on the photosensitive drum, which makes it possible to form an electrostatic latent image on the photosensitive drum without positional displacement. However, in actuality, the rotating shaft of the photosensitive drum is very slightly off-centered, and even if the photosensitive drum is rotated at a constant speed, the amount of surface movement is not constant due to adverse influence of off-centering of the shaft. Therefore, even when the exposure control is performed in synchronism with the amount of rotational movement of the photosensitive drum, an electrostatic latent image formed on the photosensitive drum may be positionally displaced.
Further, the technique disclosed in Japanese Patent Laid-Open Publication No. 2007-156194 has a problem that the use of a surface speed sensor for detecting the surface speed of the photosensitive drum increases the cost. Particularly, in a case where a surface speed sensor for detecting a scale formed on the drum surface is used, thermal deformation of the drum surface, scraping-off of the surface, etc. have influence on the result of detection, and it is difficult to cope with this influence. Besides these, there has also been proposed a method of controlling exposure in synchronism with a rotating member brought into contact with the surface of the photosensitive drum, but this cannot cope with aging of the rotating member, including scraping-off of the surface of the rotating member.